The invention relates to a cast aluminum part.
Cast aluminum parts with honed internal cylindrical running surfaces are used in engine manufacture. The cast parts have the following roughness profile as measured according to the German Standard DIN 4776:
R((k))=0.5 to 3.0 micrometers
R((nkk))=0.1 to 2.0 micrometers
R((vk))=0.5 to 5.0 micrometers.
Here, R((k)) is the core or overall roughness, R((nk) the reduced peak height, and R((vk)) the reduced groove depth.
The internal cylindrical running surfaces, e.g., the cylinder walls of internal combustion engines, have poor wear resistance. Thus, for the most part, these as-cast surfaces are treated before use to improve the wear characteristics.
The treatment can involve galvanic coating of the surfaces. On the other hand, if the cast part consists of a hypereutetic aluminum alloy such as Alusil, the surfaces are etched in order to expose the silicon-containing precipitate of the alloy. The precipitate, which has a markedly better wear resistance than aluminum, then forms the running surfaces.
In areas other than engine manufacture, e.g., tool manufacture, hard coatings are applied to substrates by Plasma Vacuum Deposition (PVD). Examples of the substances used for the coatings are TiNxe2x80x94TiAlxe2x80x94CNxe2x80x94TiCN and CrN. These coatings are very hard and consequently not well-suited for further processing as, for instance, is required for the running surfaces of cylinders.
Another drawback of conventional PVD is that the heat generated causes the substrate to undergo an unwanted thermal treatment. At higher temperatures, e.g., above 500 degrees Centigrade, the substrate may even soften or melt locally resulting in a shape change. In hardenable alloys, heating can change the microstructure to such an extent that the strength is reduced upon cooling.
Adhesion of the coating presents an additional problem. This problem arises because the thermal conductivity of the coating is substantially lower than that of the substrate. If the coated substrate is subjected to temperature fluctuations, the difference in thermal conductivity can cause the coating to loosen.
To avoid the preceding disadvantages, the International Patent Publication WO 89/03930 (Adiabatix) applies a thermally insulating coating to the surface of a cylinder in a thickness of 0.002 inch. This thickness provides a satisfactory insulating effect while improving the adhesion of the coating.
It is an object of the invention to provide a durable wear-resistant coating for an as-cast surface.
Another object of the invention is to provide an adherent wear-resistant coating for an as-cast surface.
An additional object of the invention is to provide a coating which can satisfactorily reproduce an as-cast surface.
A further object of the invention is to provide a coating which can reproduce the roughness of an as-cast surface within about 10 percent as calculated on the basis of the maximum roughness.
The preceding objects, as well as others which will become apparent as the description proceeds, are achieved by the invention.
One aspect of the invention resides in an article of manufacture. The article comprises a casting having a casting surface, and a vapor-deposited wear-resistant coating on said casting surface.
The roughness of the cylinder walls of internal combustion engines after honing is of particular significance because this defines the capacities of the lubricant pockets or reservoirs. Three roughness values are required for an exact description of a surface, namely, the core or overall roughness represented by R((k)), the reduced peak height represented by R((nk)), and the reduced groove depth represented by R((vk)). The precise effect of the lubricant pockets can be established by a friction test.
Surprisingly, it has been found that a vapor-deposited coating has the ability to reproduce a surface on which it is formed. Following vapor deposition, the grooves formed by honing are present and do not deviate greatly from their original dimensions. Hence, the machining which is normally required for hard coatings can be omitted.
Another aspect of the invention resides in an arrangement for vapor-deposition. The arrangement comprises a casting having a casting surface, and means for vapor-depositing a coating on the casting surface.
An additional aspect of the invention resides in a method of treating a casting having a casting surface. The method comprises the step of vapor-depositing a wear-resistant coating on the casting surface.
The casting may be aluminum-based and the casting surface can constitute an internal cylindrical surface of the casting. By way of example, the casting can be a cylinder block of an internal combustion engine.
The method may further comprise the step of cleaning the casting surface prior to the vapor-depositing step. This may involve ion etching the casting surface immediately before the vapor-depositing step.
The vapor-depositing step can be carried out using Plasma Vacuum Deposition and is preferably performed at a temperature lower than about 250 degrees Centigrade.
The casting surface may be a honed surface have the following roughness profile as measured according to the German Standard DIN 4776:
R((k))=0.5 to 3.0 micrometers
R((nk))=0.1 to 2.0 micrometers
R((vk))=0.5 to 5.0 micrometers.
Here, R((k)) is the core or overall roughness, R((nk)) the reduced peak height, and R((vk)) the reduced groove depth. The vapor-depositing step can be executed such that the coating has a roughness profile as follows based once again on the German Standard DIN 4776:
R((k))=0.5 to 2.8 micrometers
R((nk))=0.1 to 1.8 micrometers
R((vk))=0.5 to 4.8 micrometers.
The coating has a wearing surface, and the vapor-depositing step may be effected so that the wearing surface has a hard phase content greater than about 80 atomic percent.
The vapor-depositing step may involve forming a first coating section on the casting surface, and forming a second coating section on the first section. The first and second coating sections may form part of a single layer or, alternatively, the first coating section can constitute a base layer while the second coating section constitutes a different second layer. In either case, it is preferred for the second coating section to be formed with a hard phase.
The first coating section may be metallic and can, for example, consist of 50 atomic percent titanium and 50 atomic percent aluminum. The hard phase in the second coating section may be a nonmetallic substance.
The second coating can be formed by reacting a member of the group consisting of titanium, silicon, aluminum and chromium with an a member of the group consisting of gaseous boron, carbon, nitrogen and oxygen. The reaction is preferably carried out at a temperature of 150 to 250 degrees Centigrade.
The first and second coating sections may be produced in a gaseous atmosphere having one or more reactive components. The method can then additionally involve regulating the composition of the first coating section by controlling the concentration of at least one reactive component and/or regulating the composition of the second coating section by controlling the concentration of at least one reactive component.
The casting may have an outer peripheral wall which circumscribes the casting surface, and the method can then further comprise the step of enclosing the casting surface prior to the vapor-depositing step. It is preferred for the casting surface to be enclosed by urging a cover against the casting exclusively at the peripheral wall. If the casting is constituted by a cylinder block, the cover can be in the form of a cylinder head.